General machinery and automobiles are produced by assembling large numbers of parts. From the viewpoint of the precision requirements and production efficiency, the parts are in many cases produced through a cutting process. At this time, reduction of costs and improvement of production efficiency are required. Improvement of the machinability of the steel is also sought. In particular, conventional SUM23 and SUM24 have been developed stressing machinability. Up to now, it has been known that to improve the machinability, addition of S, Pb, or another machinability improving element is effective. However, some users sometimes avoid use of Pb due to its environmental burden. As a general direction, the amount of use is being reduced.
Up until now, when not adding Pb, the technique has been used of improving the machinability by forming inclusions such as S such as MnS becoming soft in a cutting condition. However, a similar amount of S as with the low carbon and sulfur free-machining steel SUM23 is added to so-called low carbon and lead free-machining steel SUM24L. Therefore, it is necessary to add an amount of S more than the past. However, with addition of a large amount of S, if just making the MnS coarser, not only is it necessary to obtain an MnS distribution efficient for improving the machinability, but these form starting points of fracture in rolling, forging, etc. and cause many problems in production. Further, in sulfur free-machining steel based on SUM23, the built-up edges easily form causing relief shapes at the cut surface and deterioration of the surface roughness accompanied with detachment of the built-up edges and breakoff of chips. Therefore, from the viewpoint of the machinability as well, there is the problem of a drop in precision due to the deterioration of the surface roughness. In chip disposal as well, it is considered better that the chips be able to be broken short, but with just simple addition of S, the ductility of the matrix is large, so sufficient breakage is not possible and no major improvement can be obtained.
Further, elements other than S such as Te, Bi, and P are known as elements for improving machinability, but the fact that even if improving the machinability to some extent, cracks easily occur at hot rolling or hot forging, so these are preferably made as low in content as possible is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 9-71840, Japanese Patent Application No. 2000-160284, Japanese Unexamined Patent Publication (Kokai) No. 2000-219936, and Japanese Unexamined Patent Publication (Kokai) No. 2001-329335.
Further, Japanese Unexamined Patent Publication (Kokai) No. 11-222646 proposes a method of improving chip disposal by making the establishing the presence of at least 30 sulfides of 20 μm or more alone or groups of sulfides comprised of pluralities of sulfides connected substantially linearly in lengths of 20 μm or more in an observation field of a cross-section of 1 mm2 in the rolling direction. However, the dispersion of sulfides of the submicron level most effective for machinability in practice, including the method of production, is not alluded to. Further, not much can be expected in view of the ingredients as well.
Further, Japanese Unexamined Patent Publication (Kokai) No. 11-293391 proposes a method of improving the chip disposal by making the average size of the sulfide inclusions 50 μm2 or less and establishing the presence of 750 or more sulfide inclusions per 1 mm2. However, the dispersion of sulfides of the submicron level most effective for machinability in practice is not alluded to at all like in Japanese Unexamined Patent Publication (Kokai) No. 11-222646. Further, the technology for deliberately creating this and the method for investigating this are not described either.
On the other hand, cutting tool life tends to be focused on since it has a direct effect on the production efficiency etc., but even in machinability, surface roughness is high in technical difficulty. Surface roughness is affected by the inherent properties of the cut material, so it was difficult to obtain a surface roughness equal to or greater than that of conventional steel. The surface roughness is directly linked with the performance of the part, so deterioration of the surface roughness becomes a cause of decline in part performance or an increase in the defect rate at the time of product production and is often stressed more than tool life. In this sense, conventional lead free-machining steel was superior. Compared with simple sulfur free-machining steel, it is superior not only the tool life, but also the surface roughness, so much use has been made of it for preventing a drop in part performance.
In technology relating to steel for improving the surface roughness, in general free-machining elements such as Pb and Bi are added. In addition, however, for example, as seen in Japanese Unexamined Patent Publication (Kokai) No. 5-345951, for securing a desired surface roughness by making the average size of the MnS inclusions finer to not more than 50 μm2, graphite free-machining steel superior in tool life and finished surface roughness characterized by containing graphite having an average cross-sectional area of 5 to 30 μm2 in an amount of 0.20 to 1.0% in a ferrite matrix has been seen. However, even with these techniques, it is difficult to obtain a surface roughness equal to or better than that of conventional lead free-machining steel. That is, so-called low carbon and lead free-machining steel SUM24L has been superior in surface roughness in the past. The reason is believed to be that the level of fine dispersion of inclusions defined in these only concerns grains of an average size of 3 μm or so, so homogeneous dispersion is insufficient and therefore built-up edges easily are formed and the surface roughness cannot be improved as much as that of conventional lead free-machining steel.